The present invention relates to a belt tensioner and more particularly, to an improved belt tensioner utilized in a power transmission belt drive system or a timing belt drive system of an automobile engine.
The conventional belt tensioner for an endless belt utilized in a power transmission belt drive system or a timing belt drive system comprises a support means for being stationary relative to the belt, a belt engaging means carried by the support means and being movable relative to the support means, a mechanical spring means operatively associated with the support means and the belt engaging means for urging the belt engaging means relative to the support means and against the belt with a force to tension the belt, and a friction dempening means operatively associated with the support means and the belt engaging means and dampening the movement of the belt engaging means relative to the support means in at least one direction. The dampening means has a longitudinal axis and the support means has a shaft means having a longitudinal axis and being fixed from movement relative to the belt engaging means. The belt engaging means has a portion rotatably carried by the shaft means whereby the engaging means is rotatable relative to the shaft means. The spring means has a longitudinal axis around which the spring means is wound. The shaft means is provided the dampening means and the spring means in an adjacent position along its longitudinal axis. For example, see U.S. Pat. No. 4,689,037 to Bytzek or U.S. Pat. No. 4,698,049 to Bytzek et al.
In the conventional tensioner, therefore, in order to adjust the spring force of the mechanical spring means, it is necessary to adjust simultaneously both the force along the rotation direction (the twist force of the coil spring) for biasing the belt engaging means and the longitudinal force for biasing the frictional dampening means, resulting in a difficult adjusting.
Accordingly, it has been proposed that a second mechanical spring means for urging the dampening means is interposed between the belt engaging means and the support means, and, in addition, the second mechanical spring means is positioned on the shaft means, being adjacent to the dampening means. The object of such an arrangement is that the second mechanical spring means for urging the dampening means is separated from the first mechanical spring means for urging the belt engaging means, whereby the spring force can be independently adjusted. For example, see U.S. Pat. No. 4,596,538 to Henderson.
However, some problems still remain as follows. Since an annular wavy metallic spring is utilized as a second mechanical spring means in the belt tensioner as shown in U.S. Pat. No. 4,596,538 and has the characteristics that small deformation of the spring changes the spring force remarkably, the spring force is abruptly eliminated, when the frictional face of the dampening means is worn away by long-term operation, and it is dificult to assemble the belt tensioner so that the second mechanical spring means may give the dampening means a suitable force.